1. Technical Field
This invention relates generally to self-pumped fluid bearings such as those used in light beam deflectors in beam scanning apparatus.
2. Background Art
Laser sources combined with rotating beam deflection mechanisms are known for use in reading image information (also known as input scanning), exposing or printing image information (also known as output scanning), and displaying image information. For example, holographic beam deflectors rotate a holographic disk in the path of a stationary beam of light, such as from a laser source, and the rotation causes the beam to scan.
Self-acting hydrodynamic bearings are preferred in high-speed rotating polygon and hologon laser beam deflection systems. Such a bearing supports the load (e.g., a holographic disk) by the pressure generated by a fluid flow in the bearing gap. The fluid flow is determined by the relative motion of the bearing surfaces and viscosity of the fluid. These bearings may be categorized according to the composition of the hydrodynamic fluid (gas, oil, grease, etc.). Self-acting gas bearings have the advantage that the threat of contamination of the hologon by oil or grease is avoided. At speed, a self-acting gas bearing operates on a thin gas film that develops between the bearing surfaces. The benefit is that the stiffness of the self-acting gas bearing controls the location of hologon disk 26 along spin axis 46, which is preferable to (for example) a magnetic thrust bearing, which is much more susceptible to axial perturbations of the hologon that significantly degrade the scanning accuracy of the beam scanning system.
Self-acting fluid bearings require relative movement of the bearing surfaces to generate a fluid pressure in the bearing gap. The bearing surfaces are in contact when the load is at rest and at start-up and shut-down.
Accordingly, there exists a need for a self-acting fluid bearing to support a beam deflector preferably in the form of a holographically-generated planar diffraction grating disk (i.e., a hologon) that effects smooth and precise rotation of a load over an extended series of start/stop cycles. The fluid bearing should preferably operate either vertically or tilted at an angle.
One major factor of catastrophic failures of fluid bearings appears to be the contact of the rotor assembly on the spindle and lower thrust plate during the transition from the rest state to the operating state (termed herein "lift-off"). A second factor is the tilt of the spin axis, which accentuates the load on the self-acting bearing and aggravates the impingement of the bearing surfaces during lift-off rotations of the rotor assembly.
Commonly assigned, co-pending U.S. patent application Ser. No. 08/160,121, filed in the names of T. Stephany et al. on Nov. 30, 1993, discloses a self-acting gas bearing in which the weight of the rotor assembly, which includes the hologon and other components, is reduced (i.e., unweighted) by use of opposing magnets. The frictional degradation of the lower thrust plate at start-up and touch-down is therefore reduced. Although the gas bearing described in the Stephany et al. application has been found useful in a high precision output writer application where the requirement for high speed motor operation (above 12,000 rpm) and extremely low flutter, there exists a desire for self-pumped fluid bearings that do not require the addition of unweighting magnets.